Aircraft are used in a wide variety of applications, including travel, transportation, fire fighting, surveillance and combat. Various aircraft have been designed to fill the wide array of functional roles defined by these applications. Included among these aircraft are balloons, dirigibles, traditional fixed wing aircraft, flying wings and helicopters.
One functional role that a few aircraft have been designed to fill is that of a high altitude platform. Operating from high, suborbital altitudes, such aircraft can monitor weather patterns, conduct atmospheric research and surveil a wide variety of subjects.
Three high altitude aircraft that have been constructed are the well-known Pathfinder, Centurion and Helios aircraft, which have set numerous flight records. The basic design concepts underlying these aircraft are discussed at length in U.S. Pat. No. 5,810,284, which is directed toward an unswept flying wing aircraft having a very high aspect ratio and a relatively constant chord and airfoil. While these aircraft are quite noteworthy for their long term flight potential, they do have limits in their available power and payload.
Such aircraft may be designed as flying wings that include a number of self-sufficient wing sections, each generating enough lift to support its own weight. To minimize weight, the aircraft structure is highly flexible, and is designed to withstand only relatively small torsional loads and moderate bending loads along its lateral axis (i.e., its wingspan). The aircraft's wing has little or no dihedral while on the ground. However, due to high flexibility, the large aspect ratio and the constant chord, in-flight wing loads tend to cause the wing to develop a substantial dihedral angle at the wingtips, which may not be optimal for a given wing strength. Thus, there is a tradeoff between the structural weight of the aircraft and the desirability of the wing shape.
There is an inherent relationship between an aircraft's overall airframe geometry and the design of its airfoils and control surfaces. Typical aircraft offset negative (i.e., nose-down) pitching moments through the use of tail moments (i.e., vertical forces generated on empennage horizontal surfaces and elevators, with a moment arm that is the distance from the wing center of pressure to the empennage vertical center of pressure).
To minimize the torsional loads, the Pathfinder, Centurion and Helios aircraft include “wing-mounted elevators” along a substantial portion of their trailing edges (i.e., the trailing edges of each flying wing segment). These aircraft do not include rudders or ailerons, and the wing-mounted elevators are not designed as elevons (i.e., they cannot move in contrary directions near opposite wingtips). Roll is passively controlled by the dihedral of the wing, which is developed in flight. Sideslip is also passively controlled by the dihedral of the wing. As discussed above, the allowable wing dihedral is limited by the structural strength of the wing.
Given the broad range of functions that a long-duration, suborbital platform has the potential to perform, it is desirable to design such high-altitude platforms to be capable of handling larger payloads and power demands. The platforms could be variations of existing platforms, such as larger variations of the Pathfinder, Centurion and Helios aircraft, but such platforms will likely have to handle increased bending loads along the wing as such larger aircraft have to react against dihedral-causing forces over a larger wingspan.
There exists a definite need for a multipurpose aircraft that can remain airborne for long durations. Preferably, such an aircraft should be able to operate up to very high, suborbital altitudes. Importantly, it is desirable for such an aircraft to have the capability to meet larger payload and/or power supply requirements. Furthermore, there exists a need for such an aircraft to be structurally light weight and well controlled. Various embodiments of the present invention can meet some or all of these needs, and provide further, related advantages.